The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
One of the many operational schemes for passenger cars and light trucks that is under extensive study and development in response to ever increasing consumer demands and federal mileage requirements is referred to as engine start stop (ESS). This operational scheme generally involves shutting off the gasoline, Diesel or flex fuel engine whenever the vehicle is stopped in traffic, that is, whenever the vehicle is in gear but stationary for longer than a short, relatively predictable time, such as occurs at a traffic light or in stop-and-go traffic.
While this operational scheme has a direct and positive impact on fuel consumption, it is not without engineering and operational complications. For example, since the engine output/transmission input shaft does not rotate during the stop phase, automatic transmissions relying for their operation upon pressurized hydraulic fluid provided by an engine driven pump may temporarily lose pressure and thus gear and clutch selection and control capability. This shortcoming can, however, be overcome by incorporating various hydraulic components such as accumulators or electrically driven pumps in the hydraulic control circuit at strategic locations. Such accumulators, since the are essentially passive devices, depend upon both engine operating cycles of sufficient length to fully charge the accumulator(s) and stationary engine cycles or periods of sufficient brevity that the accumulator(s) do not become discharged. Since pumps are active devices, they do not suffer from these shortcomings. Many pump designs, especially gear and rotor pumps do, however, tend to be more expensive than accumulators and, of course, require electrical supply and control components.
The cost and complexity of gear and gerotor pumps have directed attention to another type of pump, the solenoid pump. Solenoid pumps have become popular in engine start stop applications, not only for their lower cost but also because their generally somewhat limited flow and pressure output is a good match for engine start stop transmission applications.
The application is not without challenges, however, one of which is ironic. During the engine stop cycle, vehicle powertrain noise is essentially non-existent. This, of course, is typically the only time an auxiliary or supplemental hydraulic pump will be called upon to provide pressurized hydraulic fluid for the transmission. Unfortunately, solenoid pumps, which pump by cyclic energization of a coil and the resulting reciprocation of a piston, tend to create a certain amount of pulsation noise. Such pulsation noise is detectable and can be objectionable, again primarily because the vehicle is otherwise quiet during the engine stop cycle.
It is apparent, therefore, that a solenoid pump having reduced operating noise would be highly desirable. The present invention is so directed.